Farmer's lung disease, a form of hypersensitivity pneumonitis, is caused by the inhalation of organic dust and may lead to diffuse interstitial fibrosis and death. Though hypersensitivity pneumonitis represents an important group of lung diseases, there is little epidemiological data available regarding relationships between antigens, exposure and disease. All longitudinal assessments of farmer's lung have been restricted to clinically recorded cases and have not been population-based analyses. It has been suggested that individuals with circulating antibodies to farmer's lung antigens are at greater risk to developing chronic interstitial fibrosis. The long term presence of circulating antibodies is most likely the result of repeated exposure to the thermophilic antigens. The major purpose of this study is to perform a prospective population-based study to determine if individuals with a history of positive antibodies have an increased risk for developing farmer's lung, increased respiratory symptoms and a progressive decline in lung function. A second purpose of this study is to identify characteristics of the agricultural occupational environment which protect against exposure to the antigens. A third purpose of this study is to assess the economic impact of antibody status and disease status, with a specific examination of health costs and agricultural technological investments.